This invention relates to a process for the manufacture of an optical telecommunications cable.
More particularly, the invention relates to a process for the manufacture of an optical core for a telecommunications cable, wherein the optical core comprises a support consisting of a central, traction-resistant reinforcing member, a first polymeric coating layer applied around the central member, a plurality of optical fibres arranged around the first layer and a second polymeric coating layer extruded around the first layer and around the optical fibres.
Numerous processes are known for the manufacture of telecommunications cables comprising a core with optical fibres where the optical fibres, typically coated with one or more layers of acrylic resin, are completely encapsulated in thermoplastic materials.
A process is known, for example, from GB patent application 2,176,905 according to which optical fibres with acrylic coatings are forced through a die together with a central reinforcing member before being encapsulated in a thermoplastic casing of a material commercially designated xe2x80x9cHytrelo(copyright)xe2x80x9d.
This process is known as a xe2x80x9cone-shot operationxe2x80x9d, indicating that the core is formed in a single step in the extrusion die.
A method is described in GB patent no. 2,136,350 for constructing an optical core whereby a first central strength member is heated and a first thermoplastic elastomer layer is extruded on this member. A plurality of optical fibres is arranged in a helical pattern on the first layer with a planetary motion obtained from a special revolving cage whereon drums containing the fibres are arranged. A second layer of thermoplastic material is extruded around the fibres. This method requires particularly complex equipment, consisting of a revolving cage with which rotating drums of fibres are associated and also guides for directing the fibres from the cage to the extrusion head.
GB patent no. 2,113,903 describes a method for making a telecommunications cable according to which a plurality of optical conductors is encapsulated at least partially in the outer periphery of a central matrix of thermoplastic material, polyethylene for example, extruded around a central steel or nylon filament member. More particularly, the conductors are forced against the central member which is softened by heat. The pressure on the conductors is such that a predetermined depositing thereof takes place in the matrix by which they are kept apart from each other before being coated with a further second extrusion layer. US document no. 4,902,097 describes a method according to which a central reinforcing member is heated and provided with a first layer of thermoplastic elastomer. The support thus formed is heated and thereby softened to such an extent as to enable partial encapsulation therein of the optical fibres guided to the first layer through a plate with through-holes that the fibres pass through on their way to the extrusion head. A second layer of thermoplastic material is then extruded on the support and on the fibres. GB 2,303,938 discloses a method for producing an optical cable by disposing a plurality of fibers on the surface of an inner polymeric layer and embedding them into an outer polymeric layer. EP 646 819 discloses a method for reducing the PMD in fiber optic cable by imparting a controlled twist to the fiber being disposed around a coated strength member. During the manufacturing process, the strength member passes through a helically rotating closing die which applies radially inward forces on the cable core.--
The applicant has observed that the production of optical cores according to the known methods may result in attenuation of the fibre transmission properties, due to the stresses that the fibres are subjected to during production of the optical core.
For example, with the so-called xe2x80x9cone shotxe2x80x9d process, it is difficult to control the relative positions of the fibres during extrusion of the polymeric layer and the fibres are subjected to undesired and uncontrolled stresses on account of the high level of pressure exerted by the polymeric material in the extrusion head.
The Applicant has also noted that partial encapsulation of the optical fibres in a first layer of thermoplastic material extruded around a central reinforcing member and subsequently covered with a second polymeric layer, as described in GB patent 2,113,903 or U.S. Pat. No. 4,902,097, may be one of the causes of signal transmission attenuation in the fibres. In practical terms, it was seen that embedding fibres to a greater or lesser extent in a first thermoplastic layer, as described in the above patents to maintain the fibres in the desired configuration around the extrusion zone of the second layer, requires a certain mechanical compression action to be exerted on the fibres, which thus remain in the cable in a state of mechanical stress which, if high, results in attenuation of the signal. It was also observed that it is difficult to continuously keep this compression at a low level in view of the considerable lengths, running into kilometres, usually required in optical core manufacture.
The Applicant also noticed that, in the absence of a certain control over extrusion parameters, such as temperature of the molten polymer or dimension of the extruder, extrusion of the second polymeric layer onto the optical fibres arranged around the first coat may cause non-uniform distribution of pressure on the fibres, with the risk of moving the fibres from their desired configuration and of increasing attenuation of the signal when the transmission cable is in operation.
The Applicant has now found that a xe2x80x9ctightxe2x80x9d cable can be made simply and effectively, in which a plurality of optical fibres are encapsulated in a core of polymeric material consisting of at least two concentric and contiguous layers of polymer. This result may be conveniently obtained by arranging the fibres around a central support, so that they are free of the interface formed between the two contiguous polymeric layers and by controlling the extrusion parameters so that the geometric configuration of the fibres is maintained in a predetermined position.
Accordingly, one aspect of the present invention concerns a process for the manufacture of an optical core for a telecommunications cable, comprising at least one central support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally around the support and a second polymeric coating layer extruded around said first layer and around said optical fibres, which comprises the following steps:
a) arranging said optical fibres longitudinally around said first polymeric coating layer so that the optical fibres are substantially tangential to the surface of the coating and circumferentially separated from each other in a predetermined way; and
b) extruding the second layer around said first layer and around said optical fibres, maintaining a condition of substantial tangency and of circumferential separation at least as far as the exit of the extruder.
In the course of this description, the phrase xe2x80x9ccondition of substantial tangency of the fibres to the supportxe2x80x9d means a configuration wherein the fibres are placed in a position such that the interface between the two polymeric layers does not cross through the fibres. This condition generally includes both the case where the fibres are placed in substantial contact with the inner polymeric layer arranged around the central support, and the condition where the fibres are totally encapsulated in the second polymeric layer, thereby placing a thin layer of the second polymeric coat between the optical fibres and the first coating layer. This substantial tangency is typically obtained without applying any particular pressure on the fibres, either radially or longitudinally, to compress the fibres against the support or encapsulating them partially in it, as for example the mechanical type compression exerted by suitable equipment or compression obtained by winding the fibre helically under tension around the support. The expression is also used to mean that the fibre is in substantially tangential contact with the support substantially in each transversal section of the optical core.
In this description, the expression xe2x80x9ccondition of predetermined circumferential separationxe2x80x9d means that the fibres are placed at the extruder entrance at a predetermined distance from each other around the circumference of the coated central support. The xe2x80x9cmaintenance of a condition of circumferential separationxe2x80x9d at least as far as the extruder exit point means that in any case the fibres arranged longitudinally on the support must not be touching each other at the end of the extrusion process. Ideally the distance between the fibres at the end of the extrusion process will remain substantially the same as that set on entry of the fibres into the extruder. Ideally this predetermined distance is roughly the same between one fibre and the next.
A preferred aspect of this process comprises the following steps:
a) feeding the support in a longitudinal direction towards an extruder comprising an extrusion head containing a male die and a female die, where the second layer is extruded;
b) applying a plurality of fibres to said support, guiding the fibres for a predetermined distance in said direction to obtain a condition of substantial tangency at each point of contact with said support and of predetermined relative circumferential separation;
c) extruding the second layer around said first layer and around said optical fibres, maintaining a condition of substantial tangency and relative circumferential separation of the fibres as far as the exit of the extrusion head; and
d) cooling the optical core thus obtained.
According to a preferred embodiment, said extrusion head comprises a female die characterized in that it has a xe2x80x9clandxe2x80x9d with predetermined values for length xe2x80x9cLxe2x80x9d and diameter xe2x80x9cDxe2x80x9d, such as to substantially maintain a condition of substantial tangency of the fibres to the support and of relative circumferential separation between the fibres as far as the point of exit from the zone of extrusion of the second polymeric layer. In particular, the xe2x80x9cL/Dxe2x80x9d ratio values shall be between 1 and 2, and preferably between 1.3 and 1.5.
In this description, the term xe2x80x9clandxe2x80x9d of the female die is taken to mean the terminal portion of the extrusion head, typically of cylindrical shape, through which the extruded optical core is drawn.
According to a preferred aspect of this invention, said extrusion head comprises a male die characterized in that it comprises a plurality of suitable grooves arranged longitudinally along the inner wall of said male die so as to guide and maintain the optical fibres in the position of substantial tangency to the support. In particular, the grooves are arranged so that the maximum distance between two opposite grooves, close to the exit of the male die, substantially corresponds to the sum of the diameter of the support and twice the diameter of the fibres.
According to a further preferred aspect, the process is characterized in that the optical core is collected on a flywheel maintaining a ratio of the pull xe2x80x9cKxe2x80x9d on the support to the braking pull xe2x80x9ckxe2x80x9d on each fibre of between 10 and 50, the values of the said ratio being such that upon release of said pulls xe2x80x9cKxe2x80x9d and xe2x80x9ckxe2x80x9d the residual fibre elongation that must be compensated for before the fibres are subject to compression is at least about 0.02%, and preferably about 0.04%.
Again ideally the process is characterized in that the feed rate of the support is between 10 and 50 m/min.
A further aspect of this invention concerns a telecommunications cable comprising an optical core where said optical core comprises
a central support coated with a first polymeric coating layer,
a plurality of optical fibres arranged longitudinally around the support, and
a second coating layer extruded around the support and around the optical fibres, an interface being defined between said first and said second polymeric layers, characterized in that said optical fibres are free of the interface formed between the two polymeric layers and in that said fibres are arranged in an open configuration around the support.
The term xe2x80x9copen configurationxe2x80x9d intended to mean typically that the fibres are arranged around the central support without a continuous type helical winding. For example, the fibres may be arranged substantially parallel to the longitudinal axis of the central support or in an open helix configuration, i.e. with inversion of the winding direction, also known as an xe2x80x9cSZxe2x80x9d type helix. A parallel disposition of the fibres is generally preferred. Note that, in general, this definition includes configurations that involve absence of any substantial mechanical tightening of the fibres about the central support.
A further aspect of the present invention concerns a telecommunications cable comprising an optical core where said optical core comprises a central support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally around said support and a second coating layer extruded around the support and around the optical fibres, characterized in that said optical fibres have an average transmitted signal attenuation value less than a predetermined value. Ideally, the average attenuation value measured in a fibre in an optical core according to the invention is less than or equal to approximately 0.200 dB/km.
Another aspect of this invention also concerns a telecommunications cable comprising an optical core where said optical core comprises a central support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally around said support and a second coating layer extruded around the support and around the optical fibres, characterized in that the difference between the maximum and minimum average signal attenuation values in said fibres, measured during a thermal cycle ranging between 60xc2x0 C. and xe2x88x9230xc2x0 C., is less than a predetermined value. Ideally, the difference between the maximum and minimum average signal attenuation values in said fibres, measured during a thermal cycle ranging between 60xc2x0 C. and xe2x88x9230xc2x0 C., is less than 0.01 dB/km.
Yet another aspect of the invention is a method for minimizing attenuation of a signal transmitted by an optical fibre encapsulated in an optical core for a telecommunications cable, where the optical core comprises a central support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally around the support and a second coating layer extruded around the support and around the optical fibres, characterized in that said plurality. of optical fibres is arranged substantially tangentially around the support without the fibres exerting any substantial pressure on the support.
Yet a further aspect of the invention is an extruder for the manufacture of an optical core for a telecommunications cable comprising a support coated with a first polymeric coating layer, a plurality of optical fibres arranged longitudinally substantially tangentially around the support, and a second coating layer extruded around the support and around the optical fibres, said extruder comprising a male die through the axis of which the support and the fibres arranged around the latter are to be passed and a female die comprising a land through which the finished optical core is extruded, characterized in that:
a) the inner wall of the male die is provided with a plurality of longitudinal grooves adapted to receive said fibres passing through the die towards the extruder exit, the maximum distance between the walls of two diametrically opposite grooves at the die exit being substantially equal to the sum of the external diameter of the coated support and twice the diameter of a fibre, thereby ensuring a condition of substantial tangency between optical fibres and support; and
b) the ratio between the length xe2x80x9cLxe2x80x9d and the. diameter xe2x80x9cDxe2x80x9d of the land of the female die is between 1 and 2.